1. Field of the Invention
The present invention relates to a projection exposure apparatus which is used for the purpose of forming a fine pattern in production of LSIs.
2. Description of the Related Art
FIG. 46 shows a known projection exposure apparatus. This known apparatus has a fly-eye lens 3 which is disposed to confront the front side of the lamp house 1 across a mirror 2. An aperture member 4 is disposed in front of the fly-eye lens 3, and an exposure mask 8 having a desired circuit pattern is disposed to face the aperture member 4 across condenser lenses 5, 6 and a mirror 7. A wafer 10 is disposed in front of the mask 8 so as to oppose the latter across a projection lens system 9. As will be seen from FIGS. 47 and 48, the aperture member 4 has a disk-like member having a central circular aperture 4a.
Light emitted from the lamp house 1 impinges upon the fly-eye lens 3 through the mirror 2, so as to be divided into regions corresponding to the elementary lenses 3a which form the fly-eye lens 3. The light components which have passed through the elementary lenses 3a illuminate the whole region of the mask 8 through the aperture 4a of the aperture member 4, condenser lens 5, mirror 7 and the condenser lens 6. Thus, the light components from the elementary lenses 3a are superposed one on another to uniformly illuminate the mask 8. The light which has passed through the mask 8 reaches the wafer 10 past the projection lens system 9, whereby a circuit pattern is printed on the surface of the wafer 10.
It is well known that, in projection exposure apparatus of the kind described, the minimum resolution R is proportional to .lambda./NA, where .lambda. represents the wavelength of the light employed and NA represents the aperture number of the optical system. Hitherto, therefore, design of the optical system has been done in such a way as to employ a large aperture number NA so as to enhance the resolution of the projection exposure apparatus, thus coping with the current demand for higher degree of scale of integration of LSI circuits.
Increase in the aperture number NA in one hand improves the resolution, i.e., reduces the minimum resolution R bur on the other hand reduces the depth of focus DOF of the projection exposure apparatus. The focal depth DOF is proportional to .lambda./NA.sup.2 In the known projection exposure apparatus, therefore, an increase in the aperture number NA for improving the resolution is inevitably accompanied by degradation of the transfer precision due to reduction in the focal depth.
FIG. 49 illustrates a light source image formed in the pupil 9a of the projection lens system 9 when the circuit pattern of the mask 8 has parallel-line pattern of a minuteness level approximating the resolution limit. A light source image S.sub.0 is formed by the 0-order light on the center of the pupil 9a, while light source images S.sub.1 and S.sub.2 are formed by first-order light on both sides of the light source image S.sub.0. For instance, the light L.sub.0 passing through the center of the 0-order light source image S.sub.0 interferes with the lights L.sub.1 and L.sub.2 passing through the centers of the fist-Order light source images S.sub.1 and S.sub.2 so as to form a pattern on the wafer 10. The aperture number NA is given by sin .theta..sub.1, representing the angle of incidence of each of the lights L.sub.1 and L.sub.2 on the wafer 10 by .theta., Consequently, the focal depth DOF decreases as the angle of incidence of the light to the wafer 10 increases.
In order to avoid reduction in the focal depth DOF, Japanese Patent Laid-Open No. 61-91662 discloses a projection exposure apparatus which employs a ring-shaped aperture member. In this art, as shown in FIG. 50, the lights L.sub.0 to L.sub.2 passing through the centers of the respective light-source images S.sub.0 to S.sub.2 are interrupted by the aperture member Consequently, lights passing through the peripheral portions of the pupil 9 are interrupted in the region near the resolution limit, the angle .theta..sub.2 of incidence of lights to the wafer 10 is reduced to offer an appreciable improvement in the focal depth DOF. Referring to FIG. 50, however, the lights L.sub.4 and L.sub.5, among the lights L.sub.3 to L.sub.5 passing through the upper edges of the light source images S.sub.0 to S.sub.2 which are directed to regions outside the pupil 9a are interrupted by the pupil 9a. Consequently, the light L.sub.3 of the 0-order light source image S.sub.0 merely contributes to illumination of the background, without being focused in the wafer 10. Consequently, the contrast of the image is seriously impaired to deteriorate the transfer precision.
Referring to FIG. 51, there are shown lights L.sub.6 to L.sub.8 which pas through left portions of the light source images S.sub.0 to S.sub.2, among which the light L.sub.8 of the -1-order light source image S.sub.2 is interrupted by the pupil 9a. Consequently, the light L.sub.6 of the 0-order light source image S.sub.0 interferes only with the light L.sub.7 of the +1-order light source image S.sub.1, so as to form an image on the wafer 10. Similarly, the light passing through the hatched region Q.sub.0 of the 0-order light source image S.sub.0 interferes only with the light passing through he hatched region Q.sub.1 of the +1-order light source image S.sub.1, and the light passing through the right hatched region R.sub.0 of the 0-order light source image S.sub.0 interferes only with the light passing through he hatched region R.sub.2 of the -1-order light source image S.sub.2, Thus, in the hatched areas Q.sub.0, Q.sub.1, R.sub.0 and R.sub.1, one of the light of the +1-order light source image S.sub.1 and the light of the -1-order light source S.sub.2 cannot make contribution to the formation of the image.
It is assumed here that the mask 8 has a line-and-space circuit pattern having light-interrupting portions 8a and light-transmitting portions 8b having the same width, as shown in FIG. 52. In such a case, the 0-order light source image S.sub.0 has an amplitude T.sub.0 of 0.5, while the amplitudes T.sub.1 and T.sub.2 of the -1-order light source images S.sub.1 and S.sub.2 are 063/2. In FIG. 51, the amplitudes T0 to T2 are represented by thickness of the disks which indicate the light source images S.sub.0 to S.sub.2. Am optical image with large amplitude E is advantageously obtained as shown in FIG. 53 when all the light source images S.sub.0 to S.sub.2 having amplitudes T.sub.0 to T.sub.2 contribute to the formation of the image. However, when one of the lights of the +1-order light source image S.sub.1 and the light of the -1-order light source image S.sub.2 do not contribute to the formation of the image, only a small amplitude F of the optical image is obtained, resulting in an inferior contrast of the image.
FIGS. 54 and 55 show Levenson type phase shift mask. This mask has a transparent substrate 8c on which provided at a constant pitch are parallel Cr light-interrupting members 8d so that light-transmitting portion and shading portion are formed alternately. Phase shift members 8e of for example, SOG are formed in every other light-transmitting portions. When this type of phase shift mask is used, in the region where the phase shift member 8e and the transparent substrate 8c neighbor each other as viewed on the planar pattern, the light which has been transmitted both through the phase shift member 8e and the transparent substrate 8c and the light which has passed only through the transparent substrate 8c interfere with each other, so that the light intensity is reduced to zero. Therefore, when a positive resist is used, the pattern on the wafer is partly deformed as at 10b so that the patterns 10a to be formed by the light interrupting members 8d are undesirably connected, resulting in an impaired transfer accuracy.